Thermal storage device

ABSTRACT

A thermal storage device includes a reactor for generating heat by chemically reacting with ammonia; an absorber, provided with an absorbent, for storing ammonia absorbed by the absorbent; and a connection line for connecting the reactor and absorber to each other and moving ammonia between the reactor and absorber. The reactor has a thermal storage member provided so as to cover an outer peripheral portion of the catalyst, a porous sheet provided so as to cover an outer peripheral portion of the thermal storage member, and a casing for enclosing the thermal storage member and porous sheet. One end of the connection line penetrates the casing and opens to the porous sheet.

TECHNICAL FIELD

The present invention relates to a thermal storage device for warming up a catalyst provided in an exhaust system of an internal combustion engine of a vehicle.

BACKGROUND ART

A vehicle has an exhaust system provided with a catalyst and the like in order to purify environmental pollutants (HC, CO, NOx, and the like) contained in an exhaust gas discharged from its engine. The catalyst has an optimal temperature (active temperature) for activating its purification performance. At the time of starting the engine, the exhaust gas has a low temperature, which takes time to reach the active temperature of the catalyst. In order for the temperature in the catalyst to rise to the active temperature in a short time when the temperature of the exhaust gas is low at the time of starting the engine and the like, a device for warming up the catalyst is necessary. As such a device, thermal storage devices utilizing the reaction heat of chemical reactions have been known. The thermal storage devices cut down fuel consumption losses and then perform warm-up. Patent Literature 1 discloses a catalyst warm-up device in which a thermal storage member is arranged on the outside of a catalyst, while the reaction heat of a chemical reaction of the thermal storage device is utilized for warming up the catalyst.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Patent Application Laid-Open No. S59-208118

SUMMARY OF INVENTION Technical Problem

The catalyst is supported by a honeycomb base disposed within an exhaust pipe and required to be protected against shocks at the time when the vehicle is running. Therefore, when the thermal storage member is arranged on the outside of the catalyst as in Patent Literature 1, a mat or the like is provided between the catalyst and thermal storage member in general and absorbs the shocks. When such a mat is provided between the catalyst and thermal storage member, however, heat generated in the thermal storage member is transmitted to the catalyst through the mat, which lowers the efficiency of heat transfer to the catalyst. When the mat or the like is not provided between the catalyst and thermal storage member, the heat transfer efficiency does not lower, but shocks occurring when the vehicle is running are hard to absorb.

It is therefore an object of the present invention to provide a thermal storage device which can prevent the thermal transfer efficiency from lowering and absorb shocks at the time when the vehicle is running.

Solution to Problem

The present invention provides a thermal storage device for warming up a catalyst arranged in an exhaust system of an internal combustion engine of a vehicle, the thermal storage device including a reactor for generating heat by chemically reacting with ammonia; an absorber, provided with an absorbent, for storing ammonia absorbed by the absorbent; and a connection line for connecting the reactor and absorber to each other and moving ammonia between the reactor and absorber; the reactor having a thermal storage member provided so as to cover an outer peripheral portion of the catalyst, a porous sheet provided so as to cover an outer peripheral portion of the thermal storage member, and a casing for enclosing the thermal storage member and porous sheet; one end of the connection line penetrating the casing and opening to the porous sheet.

An exhaust system of an internal combustion engine of a vehicle is provided with a catalyst for purifying an exhaust gas and provided with a thermal storage device for warming up the catalyst. The present invention includes a reactor for generating heat by chemically reacting with ammonia and an absorber for storing ammonia absorbed by an absorbent. The reactor and absorber are connected to each other through a connection line in order to move ammonia between the reactor and absorber. The reactor has a thermal storage member and a porous sheet, which are enclosed in a casing. The thermal storage member is provided so as to cover an outer peripheral portion of the catalyst and warms up the catalyst. The porous sheet is provided so as to cover an outer peripheral portion of the thermal storage member and constitutes passages for ammonia. One end of the connection line opens to the porous sheet, so that ammonia flows into the latter from the connection line. Ammonia having flown therein is uniformly dispersed by the porous sheet, so as to be supplied to the thermal storage member. The porous sheet produces a stress buffer effect. Therefore, when the thermal storage member volume is increased by ammonia supplied thereto, the porous sheet inhibits the thermal storage member from deforming radially of the exhaust pipe. Inhibiting the thermal storage member from deforming radially of the exhaust pipe restrains the heat transfer distance from becoming longer. The porous sheet also absorbs shocks at the time when the vehicle is running and the like. As a result of these, the heat transfer efficiency of the heat generated in the thermal storage member to the catalyst can be prevented from lowering, while the shocks at the time when the vehicle is running and the like can be absorbed.

The reactor may further have a heat-insulating member enclosed in the casing and provided so as to cover an outer peripheral portion of the porous sheet.

In the reactor, the thermal storage member, porous sheet, and heat-insulating member are enclosed in the casing. The heat-insulating member is provided so as to cover an outer peripheral portion of the porous sheet and prevents heat from escaping to the outside. Therefore, the heat-insulating member absorbs shocks at the time when the vehicle is running and the like, while inhibiting the thermal storage member from deforming radially of the exhaust pipe when the thermal storage member volume is increased by ammonia supplied thereto. As a result of these, the heat transfer efficiency of the heat generated in the thermal storage member to the catalyst can further be prevented from lowering, while the shocks at the time when the vehicle is running and the like can be absorbed more.

The porous sheet may be formed from stainless steel or ceramic. In this case, the porous sheet serving as passages for ammonia is not corroded by ammonia, which enables the porous sheet to produce a stress buffer effect.

Advantageous Effects of Invention

The present invention can provide a thermal storage device which can prevent the thermal transfer efficiency from lowering and absorb shocks at the time when the vehicle is running.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic structural diagram of an exhaust purification system provided with the thermal storage device in accordance with an embodiment of the present invention; and

FIG. 2 is a sectional view of a reactor of the thermal storage device in accordance with the embodiment.

DESCRIPTION OF EMBODIMENTS

In the following, an embodiment of a thermal storage device in accordance with the present invention will be explained in detail with reference to the drawings. In the drawings, the same or equivalent constituents will be referred to with the same signs while omitting their overlapping descriptions.

In this embodiment, the thermal storage device in accordance with the present invention is employed in a thermal storage device implemented in an exhaust purification system provided in an exhaust system of an engine (internal combustion engine) of a vehicle. The exhaust purification system in accordance with this embodiment includes DOC (Diesel Oxidation Catalyst), SCR (Selective Catalytic Reduction), and ASC (Ammonia Slip Catalyst) as catalysts, DPF (Diesel Particulate Filter) as a filter, and a chemical thermal storage device for warming up the catalysts.

With reference to FIGS. 1 and 2, an exhaust purification system 1 in accordance with this embodiment will be explained. FIG. 1 is a schematic structural diagram of the exhaust purification system provided with the thermal storage device in accordance with this embodiment. FIG. 2 is a sectional view of the reactor of the thermal storage device in accordance with this embodiment.

The exhaust purification system 1 has a diesel oxidation catalyst (DOC) 4, a diesel particulate filter (DPF) 5, a selective catalytic reduction (SCR), and an ammonia slip catalyst (ASC) 7. The DOC 4, DPF 5, SCR 6, and ASC 7 are arranged in an order of the DOC 4, the DPF 5, the SCR 6, and the ASC 7 from the upstream side to downstream side of an exhaust pipe 3 connected to the exhaust side of an engine 2.

The DOC 4 oxidizes HC, CO, and the like contained in the exhaust gas. The DPF 5 collects and removes PM (Particulate Matter) contained in the exhaust gas. The SCR 6 supplies ammonia (NH₃) or urea water to the upstream side within the exhaust pipe 3 from an injector 6 a, so that NOx contained in the exhaust gas and ammonia chemically react with each other, thereby reducing and purifying NOx. Urea water is hydrolyzed into ammonia. The ASC 7 oxidizes ammonia having flowed through the SCR 6 to the downstream side thereof.

The catalysts 4, 6, 7 have respective temperature regions where they can exhibit purification performances for environmental pollutants (i.e., active temperatures). For example, the lower limit for active temperature of the DOC 4 is about 150° C. Immediately after starting the engine 2 and the like, the temperature of the exhaust gas just after being discharged from the engine 2 is relatively low, i.e., about 100° C. In order for the catalysts 4, 6, 7 to exhibit their purification performances even immediately after starting the engine 2 and the like, it is necessary for the temperatures in the catalysts 4, 6, 7 to rise promptly to their active temperatures. Therefore, the exhaust purification system 1 has a thermal storage device 8 for warming up the catalysts. The exhaust purification system 1 is provided with a temperature sensor for detecting the temperature of the exhaust gas discharged from the engine 2 (or temperatures of the catalysts).

The thermal storage device 8 is a chemical thermal storage device which warms up the catalysts without necessitating external energy such as electric power. The thermal storage device 8 usually stores the heat of the exhaust gas (exhaust heat) and, when necessary, uses the stored heat, so as to warm up the catalysts. The thermal storage device 8 warms up the DOC 4, which is a catalyst located on the upstream side in the exhaust pipe 3. The thermal storage device 8 includes a reactor 9, an absorber 10, a connection line 11, and an open-close valve 12.

The reactor 9 is provided all over the surface of an outer peripheral portion of the DOC 4. The reactor 9 has a thermal storage member (reactive member) which chemically reacts with ammonia. In the reactor 9, ammonia and the thermal storage member react with each other, thereby generating heat. Examples of the thermal storage member include metal chlorides, metal bromides, and metal iodides, specific examples of which include MgCl₂, CaCl₂, NiCl₂, ZnCl₂, and SrCl₂. The structure of the reactor 9 will be explained later in detail.

The absorber 10 incorporates therein activated carbon as an absorbent which physically absorbs ammonia. The absorber 10 stores ammonia by causing the activated carbon to physically absorb ammonia and releases ammonia by separating it from the activated carbon.

The connection line 11 connects the reactor 9 and absorber 10 to each other. The connection line 11 is a conduit for moving ammonia between the reactor 9 and absorber 10. The open-close valve 12 is arranged in the connection line 11. When the open-close valve 12 is opened, ammonia can move between the reactor 9 and absorber 10 through the connection line 11. The open-close valve 12 is opened and closed under the control of an ECU (Electronic Control Unit) (not depicted), which controls the engine 2, or the like.

With reference to FIG. 2, the structure of the reactor 9 will be explained. As mentioned above, the reactor 9 is provided so as to cover the whole surface of the outer peripheral portion of the DOC 4 through the exhaust pipe 3. That is, the reactor 9 is arranged so as to cover the outer peripheral portion of the DOC 4 as a whole indirectly through the exhaust pipe 3. The DOC 4 has a honeycomb base. The honeycomb base is arranged in the thin exhaust pipe 3 having an annular cross section. The honeycomb base supports the catalyst. The reactor 9 has a plurality of thermal storage members 9 a, a porous sheet 9 b, a heat-insulating member 9 c, and a casing 9 d. The plurality of thermal storage members 9 a, porous sheet 9 b, heat-insulating member 9 c, and casing 9 d are arranged on the outside of the DOC 4.

A pair of annular plates (not depicted) are provided orthogonal to the outer periphery of the exhaust pipe 3 on the outer periphery of the exhaust pipe in a portion where the DOC 4 is located. One annular plate is arranged about a position where the upstream end of the DOC 4 is located. The other annular plate is arranged about a position where the downstream end of the DOC 4 is located. The exhaust pipe 3 and the pair of annular plates define a space for containing the thermal storage members 9 a, porous sheet 9 b, and heat-insulating member 9 c.

Each thermal storage member 9 a is in a solid tablet form. When arranged on the outside of the exhaust pipe 3, as illustrated in FIG. 2, the plurality of thermal storage members 9 a as a whole exhibit a substantially annular ring form in a cross section taken along a plane orthogonal to the outer periphery of the exhaust pipe 3. The plurality of thermal storage members 9 a are arranged in a region between the pair of annular plates in the outer periphery of the exhaust pipe 3 so as to align longitudinally and circumferentially of the exhaust pipe 3. The plurality of thermal storage members 9 a are in contact with the outer periphery of the exhaust pipe 3. Therefore, the plurality of thermal storage members 9 a arranged in contact with the outer periphery of the exhaust pipe 3 warm up the DOC 4 through the exhaust pipe 3. Since the exhaust pipe 3 is thin, the plurality of thermal storage members 9 a warm up the DOC 4 substantially directly. The plurality of thermal storage members 9 a are arranged so as to cover the outer periphery of the exhaust pipe 3 directly. That is, the plurality of thermal storage members 9 a are arranged so as to cover the outer peripheral portion of the DOC 4 as a whole indirectly through the exhaust pipe 3.

The volume of thermal storage members 9 a increases when ammonia infiltrates therein. The plurality of thermal storage members 9 a are arranged with gaps between the thermal storage members 9 a adjacent to each other longitudinally of the exhaust pipe 3 so that the volume of thermal storage members 9 a do not increase radially but longitudinally of the exhaust pipe 3. That is, when containing no ammonia, the thermal storage members 9 a in contact with each other, whose number is the same as those longitudinally arranged between a pair of annular plates, have a total length shorter than the distance between the pair of annular plates.

The porous sheet 9 b is in the form of a thin sheet. When arranged on the outside of the exhaust pipe 3, the porous sheet 9 b has a substantially cylindrical form. Therefore, the porous sheet 9 b as a whole exhibits a substantially annular ring form in a cross section taken along a plane orthogonal to the outer periphery of the exhaust pipe 3 as illustrated in FIG. 2. The porous sheet 9 b is arranged between the pair of annular plates so as to extend along the outer peripheries of the plurality of thermal storage members 9 a placed along the outer periphery of the exhaust pipe 3. The porous sheet 9 b is in the form of a fine filter (net) and constitutes passages for ammonia. For constituting the passages for ammonia, it is necessary for the porous sheet 9 b to be formed from a material which is not corroded by ammonia. For producing a stress buffer effect, it is also necessary for the porous sheet 9 b to be formed from an elastically deformable material (flexible material). Examples of materials satisfying these conditions include stainless steel and ceramics. The porous sheet 9 b is arranged so as to directly cover the outer peripheral portions of the plurality of thermal storage members 9 a exhibiting a substantially cylindrical form as a whole.

The heat-insulating member 9 c has a predetermined thickness. When arranged on the outside of the exhaust pipe 3, the heat-insulating member 9 c has a substantially cylindrical form. Therefore, the heat-insulating member 9 c exhibits a substantially annular ring form in a cross section taken along a plane orthogonal to the outer periphery of the exhaust pipe 3 as illustrated in FIG. 2. The heat-insulating member 9 c is arranged between the pair of annular plates so as to extend along the outer periphery of the porous sheet 9 b. The heat-insulating member 9 c is relatively hard in order to inhibit the volume of thermal storage members 9 a from increasing radially of the exhaust pipe 3. For producing a stress buffer effect, it is also necessary for the heat-insulating member 9 c to be formed from an elastically deformable material. The heat-insulating member 9 c is also porous in this embodiment. It is not necessary for the heat-insulating member 9 c to be porous. The heat-insulating member 9 c is arranged so as to directly cover the outer peripheral portion of the porous sheet 9 b.

The casing 9 d is arranged between the pair of annular plates so as to extend along the outer periphery of the heat-insulating member 9 c. The casing 9 d has both end portions welded to the pair of annular plates, respectively. Therefore, the space defined by the casing 9 d, exhaust pipe 3, and pair of annular plates is closed and encloses therein the thermal storage members 9 a, porous sheet 9 b, and heat-insulating member 9 c. The thermal storage members 9 a are enclosed in the closed space and thus can repeatedly chemically react with ammonia. The outer periphery of the casing 9 d is provided with a flange 13 for attaching the exhaust pipe and the like to a vehicle body.

The connection line 11 has one end 11 a which penetrates the casing 9 d and heat-insulating material 9 c and opens to the outer periphery side of the porous sheet 9 b. Therefore, when ammonia is released from the absorber 10 while the open-close valve 12 is open, ammonia flows into the porous sheet 9 b from the one end 11 a of the connection line 11.

Actions of the thermal storage device 8 (the reactor 9 in particular) in the exhaust purification system 1 will now be explained. When the vehicle is stopped (the engine 2 is stopped), the open-close valve 12 is closed. Therefore, even when separated from activated carbon in the absorber 10, ammonia is not supplied to the reactor 9 through the connection line 11.

When the temperature of the exhaust gas discharged from the engine 2 is lower than a predetermined temperature (a temperature lower than the active temperature of the DOC 4 and the like) after starting the engine 2, the open-close valve 12 is opened under the control of the ECU immediately after starting the engine 2, for example, whereby ammonia is supplied to the reactor 9 through the connection line 11. At this time, the pressure of the absorber 10 is higher than that of the reactor 9, so that ammonia moves toward the reactor 9.

In the reactor 9, ammonia flows into the porous sheet 9 b from one end 11 a of the connection line 11. The porous sheet 9 b diffuses ammonia uniformly. The uniformly diffused ammonia is supplied equally to the plurality of thermal storage members 9 a disposed on the outer periphery of the exhaust pipe 3. Each thermal storage member 9 a (made of MgCl₂, for example) chemically reacts with the supplied ammonia, so as to occlude ammonia, thereby generating heat. When each thermal storage member 9 a is made of MgCl₂, MgCl₂ and ammonia form a coordinate bond. That is, a reaction from the left side to right side in the following reaction formula occurs. The heat generated from each thermal storage member 9 a is transmitted to the DOC 4 through the thin exhaust pipe 3. As a result, the temperature of each of the catalysts such as the DOC 4 rises to the active temperature suitable for purifying the environmental pollutants in the exhaust gas.

MgCl₂+6NH₃

MgCl₂.6.NH₃+ΔH

ΔH=67 kJ/mol (NH₃)

In the reactor 9, the heat-insulating member 9 c makes it hard for heat to escape to the outside, whereby the heat generated in each thermal storage member 9 a is transmitted to the inside (the DOC 4 side). In the reactor 9, the volume of each thermal storage member 9 a is increased by ammonia supplied thereto. At this time, each thermal storage member 9 a is inhibited from deforming radially of the exhaust pipe 3 by the hard heat-insulating member 9 c (and further by the stress buffer effect produced by the porous sheet 9 b and heat-insulating member 9 c), so as to deform longitudinally of the exhaust pipe 3. This restrains the heat transfer distance from each thermal storage member 9 a from becoming longer. In the reactor 9, each thermal storage member 9 a is arranged on the outer periphery of the thin exhaust pipe 3, thereby substantially directly warming up the DOC 4 (catalyst). As a result of these, the efficiency of heat transfer from each thermal storage member 9 a to the DOC 4 (catalyst) can be prevented from lowering.

When the temperature of the exhaust gas discharged from the engine 2 is higher than a predetermined temperature (a temperature higher than the active temperature of the DOC 4 and the like), the exhaust heat of the exhaust gas is imparted to the thermal storage members 9 a in the reactor 9. This separates ammonia from the thermal storage members 9 a, thereby yielding high-temperature ammonia. That is, a reaction from the right side to left side in the above-mentioned reaction formula occurs. The separated ammonia returns from the reactor 9 to the absorber 10 through the connection line 11. That is, the separated ammonia passes through the porous sheet 9 b from the thermal storage members 9 a and then returns to the absorber 10 through the connection line 11. At this time, the pressure of the reactor 9 is higher than that of the absorber 10, so that ammonia moves toward the absorber 10. In the absorber 10, the activated carbon physically absorbs and stores ammonia.

When a vehicle is running, shocks caused by the running vehicle are absorbed by the stress buffer effect produced by the porous sheet 9 b and heat-insulating member 9 c. Therefore, the DOC 4 and thermal storage members 9 a can be protected against the shocks caused by the running vehicle. In particular, when vehicles collide, large shocks caused by the collision are absorbed by the stress buffer effect produced by the porous sheet 9 b and heat-insulating member 9 c, whereby the DOC 4 and the like can be prevented from being damaged.

In the thermal storage device 8 (the reactor 9 in particular) of the exhaust purification system 1, the thermal storage members 9 a are disposed on the outer peripheral portion of the DOC 4 (the outer periphery of the thin exhaust pipe 3), while the porous sheet 9 b is provided all over the outer periphery of the thermal storage members 9 a. This can prevent the heat transfer efficiency of the heat generated in the thermal storage members 9 a to the DOC 4 (catalyst) from lowering and absorb shocks at the time when the vehicle is running and the like. As a result, the efficiency of heating by the thermal storage device 8 does not lower, so that heat loss can be suppressed, whereby the temperature of the DOC 4 and the like rapidly reaches the active temperature.

In the thermal storage device 8, the heat-insulating member 9 c is provided all over the outer periphery of the porous sheet 9 b. This can further prevent the heat transfer efficiency of the heat generated in the thermal storage members 9 a to the DOC 4 from lowering and absorb shocks more at the time when the vehicle is running and the like. In particular, since the heat-insulating member 9 c is hard, the volume of thermal storage members 9 a increased by ammonia supplied thereto can be inhibited from deforming radially of the exhaust pipe 3, whereby the heat transfer distance can be prevented from becoming longer.

In the thermal storage device 8, the porous sheet 9 b is formed from stainless steel or ceramic. Therefore, the porous sheet 9 b serving as passages for ammonia is not corroded by ammonia, which enables the porous sheet 9 b to produce a stress buffer effect.

While an embodiment in accordance with the present invention is explained in the foregoing, the present invention can be carried out in various modes without being restricted to the above-mentioned embodiment.

For example, while the present invention is employed in an exhaust purification system including the DOC, SCR, and ASC as catalysts and further the DPF as a filter in this embodiment, its applicable exhaust purification systems are not limited thereto. The present invention is also employable in other exhaust purification systems having various structures as long as they include at least a catalyst and a thermal storage device for warming up the catalyst. The engine is not limited to diesel engines, but may be any of gasoline engines and the like.

While the DOC 4 is a catalyst to warm up and is arranged with the reactor 9 of the thermal storage device 8 in this embodiment, the reactor 9 may be placed at other locations according to catalysts to warm up. When the catalyst to warm up is the SCR 6, for example, the SCR 6 may be arranged with the reactor 9. In this case, the reactor 9 is provided so as to cover an outer peripheral portion of the SCR 6.

The material for the porous sheet 9 b is stainless steel or ceramic in this embodiment but is not limited thereto. The porous sheet 9 b may also be made of any of materials other than stainless steel and ceramics as long as they are elastically deformable without being corroded by ammonia.

While the thermal storage device 8 is provided with the heat-insulating member 9 c in this embodiment, it is not necessary for the thermal storage device 8 to be provided with the heat-insulating member 9 c as in this embodiment. When not provided with the heat-insulating member 9 c, the thermal storage device 8 may include porous sheets arranged in two layers on inner and outer sides. In this case, the inner and outer porous sheets may be soft and hard in order to produce a stress buffer effect and a deformation suppression effect, respectively.

The reactor 9 (the thermal storage members 9 a, porous sheet 9 b, and heat-insulating member 9 c) is arranged so as to cover the outer peripheral portion of the catalyst (DOC 4) as a whole in this embodiment, but is not restricted thereto. The reactor 9 may also be arranged so as to cover a portion of the outer peripheral portion of the catalyst.

INDUSTRIAL APPLICABILITY

The present invention can be utilized in a thermal storage device for warming up catalysts provided in exhaust systems of internal combustion engines (e.g., diesel engines).

REFERENCE SIGNS LIST

1: exhaust purification system; 2: engine; 3: exhaust pipe; 4: diesel oxidation catalyst (DOC); 5: diesel particulate filter (DPF); 6: selective catalytic reduction (SCR); 6 a: injector; 7: ammonia slip catalyst (ASC); 8: thermal storage device; 9: reactor; 9 a: thermal storage member; 9 b: porous sheet; 9 c: heat-insulating member; 9 d: casing; 10: absorber; 11: connection line; 12: open-close valve; 13: flange. 

1. A thermal storage device for warming up a catalyst arranged in an exhaust system of an internal combustion engine of a vehicle, the thermal storage device comprising: a reactor for generating heat by chemically reacting with ammonia; an absorber, provided with an absorbent, for storing ammonia absorbed by the absorbent; and a connection line for connecting the reactor and absorber to each other and moving ammonia between the reactor and absorber; wherein the reactor has a thermal storage member provided so as to cover an outer peripheral portion of the catalyst, a porous sheet provided so as to cover an outer peripheral portion of the thermal storage member, and a casing for enclosing the thermal storage member and porous sheet; and wherein one end of the connection line penetrates the casing and opens to the porous sheet.
 2. The thermal storage device according to claim 1, wherein the reactor further has a heat-insulating member enclosed in the casing and provided so as to cover an outer peripheral portion of the porous sheet.
 3. The thermal storage device according to claim 2, wherein the porous sheet is formed from stainless steel or ceramic.
 4. The thermal storage device according to claim 1, wherein the porous sheet is formed from stainless steel or ceramic. 